These types of reflector antennas are chiefly used because they are straightforward and inexpensive to manufacture. They also provide greater antenna efficiency and lower side lobes in the radiation pattern than is the case when the feed has to be supported by diagonal struts. The drawback with the latter configuration is that the main reflector becomes blocked.
A self-supported feed is also easily accessible from the back of the reflector, thus is frequently selected when it is best to locate the transmitter and/or the receiver there. This also reduces the loss that occurs when the RF energy has to be routed through a cable along one of the support struts.
A. Chlavin, "A New Antenna Feed Having Equal E and H-Plane Patterns", IRE Trans. Antennas Propagat., Vol.AP-2, pp.113-119, July 1954, describes a reflector antenna with a self-supporting feed. However since this antenna uses a waveguide with a rectangular cross-section, it can only transmit or receive waves with one particular linear polarization.
C. C-Cutler, "Parabolic-antenna design for microwaves", Proc.IRE.Vil.35, pp. 1284-1294, November 1947, describes a dual polarized reflector antenna with two variants of a self-supporting feed, called the "ring focus waveguidecup" feeds, respectively. A circular waveguide is used in these two feeds with a reflecting object in front of the waveguide opening. This reflector is respectively shaped as a flat disc and a cup. Both of these prior art feeds produce high cross polarization within the main lobe of the radiation pattern, and high sidelobes and spillover contrary to the arrangement provided in the present invention.
The main purpose of the present invention is to design a reflector antenna which has dual polarization with low cross polarization within the main lobe of the radiation pattern and low sidelobes. Dual polarization means that the antenna is capable of receiving or transmitting simultaneously two separate RF signals with orthogonal, linear or circular polarization. To enable this, the waveguide must have an almost circular or square cross-section.
This present objective can be achieved by a design which is in accordance with the claim 1 of this application. Further details about the invention are given in claims 2-8 of this application.